1. Field of the Invention
The present invention relates to a power semiconductor device, and more particularly to a semiconductor device having a low ON-state voltage and a large load short-circuit resistivity.
2. Description of the Prior Art
Insulated gate bipolar transistor (hereinafter, referred to as "IGBT") is widely used in the various field such as power source, inverter and the like, since the insulated gate bipolar transistor is a voltage driven type element and requires a simple driving circuit comparing to a current driven type bipolar transistor. Further, the insulated gate bipolar transistor has been widely and rapidly used in a field of comparatively high voltage, since the loss in normal operation is smaller than that of a MOSFET which is also a voltage driven type element.
In order to protect a device having an IGBT from breakdown, it is required for the IGBT to be broken down for a certain period of short-circuit state even when the device is operated with the load short-circuited. As a means against this problem, an over-current protecting circuit is provided in the IGBT (Japanese Patent Application Laid-Open No. 2-66975 (1990). The circuit comprises a first main terminal T.sub.1, a second main terminal T.sub.2, a control terminal T.sub.3, a main IGBT element S.sub.1 having a collector, an emitter and a gate connected to the first main terminal T.sub.1, the second main terminal T.sub.2 and the control terminal T.sub.3 respectively, a sub-IGBT element S.sub.2 having a smaller capacity than the capacity of the main IGBT element S.sub.1 and having a collector connected to the first main terminal T.sub.1, an emitter connected to the second main terminal T.sub.2 through a resistance R and a gate connected to the control terminal T.sub.3 respectively, a mosfet element S.sub.3 having a gate connected to the junction point of the sub-IGBT element S.sub.2 and the resistance R, a source and a drain connected to the second main terminal T.sub.2 and the control terminal T.sub.3 respectively. The operation of the circuit is as follows. When a load connected between the first main terminal and the second main terminal is short-circuited, an over-current approximately proportional to the current capacity flows between the first and the second terminals, that is, between the main IGBT element S.sub.1 and the sub-IGBT element S.sub.2. As the current flowing through the sub-IGBT element S.sub.2 increases due to the over-current, the voltage drop consequently increases due to the resistance R. When the voltage drop due to the resistance R exceeds the threshold of the MOSFET element S.sub.3, the MOSFET element S.sub.3 is brought into ON-state. When the MOSFET element S.sub.3 is brought into ON-state, the circuit between the control terminal T.sub.3 and the second main terminal T.sub.2 is short-circuited through the ON-state resistance of the MOSFET element S.sub.3. Consequently the gate voltages in the main IGBT element S.sub.1 and in the sub-IGBT element S.sub.2 decrease, the over-currents flowing through the main IGBT element S.sub.1 and through the sub-IGBT element S.sub.2 are suppressed or cut off to protect the device against break-down.
In a conventional technology, the main IGBT element S.sub.1 and the sub-IGBT element S.sub.2 are composed of most of or a part of small IGBT units, so-called cells, (for example, several tens of the cells) which are formed in a single semiconductor substrate by several thousands to several ten thousands in number at a time. This is because the ratio of currents flowing through the main IGBT element S.sub.1 and the sub-IGBT element S.sub.2 is accurately designed and the ratio of the currents does not change depending on the substrate temperature. Therefore, the surface structure and the cross-sectional structure of the main IGBT element S.sub.1 through which main current flows are the same as those of the sub-IGBT element S.sub.2 for detecting current. On account of this, there is a disadvantage in that the ON-state voltage increases and the loss consequently increases when the short-circuit resistivity is kept high, and the load short-circuit resistivity (the time period from entering of IGBT element into ON-state to break-down of the IGBT element with the load short-circuited) decreases when the ON-voltage is lowered in order to decrease the loss. This phenomena will be described below in detail.
The output characteristic of an IGBT, shown with taking the voltage between collector-emitter in abscissa and the output current in ordinate, can be divided into three regions, (I) a region where the current hardly flows until the p-n junction in the collector side is forward-biased against the voltage between collector-emitter, (II) a region where the current linearly increases as the voltage increase, (III) a region where the current hardly increases and is saturated regardless of increasing of the voltage. The IGBT is generally used in the region (II) where the current linearly increases as the voltage increase. On the other hand, the voltage drop caused in an IGBT at rated current is called as ON-state voltage. And the lower the ON-state voltage is, the lower the loss of the IGBT is. And the larger the gradient in the region (II) where the current linearly increases is, the larger the output current is and the smaller the ON-state voltage is. The output current I.sub.c consists of electron current and hole current. Since the hole current is the collector current in the p-n-p transistor with the electron current as the base current, the hole current, putting the electron current as I.sub.e, is expressed by multiplying the amplification factor .beta. of the p-n-p transistor to the electron current I.sub.e, that is, I.sub.e .times..beta.. Then the output current I.sub.c can be expressed as the following equation. EQU I.sub.c =(1+.beta.).times.I.sub.e. (1)
The output current I.sub.c is proportional to the electron current I.sub.e. Further, the electron current I.sub.e is proportional to the conductance gm which is given by the following equation. EQU gm=W/L.times..mu..sub.CH .multidot..epsilon..sub.SiO2 /t.sub.ox .times.(V.sub.G -V.sub.T), (2)
where W: gate width, L: gate length, t.sub.ox : thickness of oxide film, V.sub.g : gate voltage, V.sub.T : threshold voltage, .mu..sub.CH : mobility of channel layer, .epsilon..sub.SiO2 : dielectric constant of the oxide film.
It can be understood from this equation that in a case of a constant element area and a constant gate voltage, the value of gm becomes larger, that is, the electron current flows more and consequently the ON-state voltage becomes lower as the gate width is longer, the gate length being shorter, the oxide film being thinner, the threshold voltage being lower. The saturated current also consists of a constant electron current and a constant hole current flowing when the channel is pinched off. Since the hole current is the collector current of the p-n-p transistor with the electron current as the base current, the saturated current is also proportional to the electron current similar to the output current.
Discussion will be made on a case where the cells composing the main IGBT element S.sub.1 and the sub-IGBT element S.sub.2 shown in FIG. 15 are the same surface structure and the same cross-sectional structure. In order to decrease the loss in the main IGBT element S.sub.1 conducting the main current, a cell is designed to realize a low ON-state voltage in such a way as to increase the value of gm. By doing so, it is natural that the saturated current becomes large. The saturated current means the maximum current flowing in the IGBT element. When the saturated current is large, the joule heat generated proportional to the product of the voltage and the current becomes large. The temperature rise due to this heat makes the ON-state resistance large, finally the parasitic thyristor is latched up and the main IGBT element S.sub.1 is broken by overheat. That is, the load short-circuit resistivity is lowered. As described above, the low ON-state voltage and the high load short-circuit resistivity are inconsistent with each other. It is impossible to realize the both at a time.
On account of this reason mentioned above, in a practical design of assigning priority to the over-current protection, the saturated current is suppressed by means of making the value of gm so small that the main IGBT element S.sub.1 conducting the main current and the sub-IGBT element S.sub.2 conducting the current for detecting over-current have a load short-circuit resistivity larger than the time period from the time the gate receives an ON-state signal to the time the MOSFET element S.sub.3 turns into an ON-state. Therefore, the ON-state voltage has a performance penalty.